In a fabrication process of a semiconductor device, a photoresist is separated and removed from a substrate after its role has been filled in a step of etching or the like. However, such a photoresist film remains unremoved as a permanent film if it is used as a surface-protecting film or a layer insulation film. Therefore, it is required to have electrically and mechanically high characteristics or properties. Further, such a film also must resist a high temperature applied in the fabrication process of the semiconductor device.
In recent years, a film formed of a photosensitive polyimide resin has been used in such an application. The polyimide resin is excellent in electrical and mechanical characteristics, and heat resistance and is used as a surface-protecting film or layer insulation film for a semiconductor device, or the like. In order to selectively provide a film of a polyimide resin on minute parts of the semiconductor device, it is convenient to use a photosensitive polyimide resin.
As a process for selectively providing the polyimide resin film on the minute parts, there has heretofore been known a process in which a polyimide resin is coated on the whole surface of a semiconductor device to form a film, a pattern is formed on the film by means of a photoresist, and the polyimide resin film is etched with hydrazine or the like. However, this process is complicated in process and requires to use an etchant high in toxicity.
On the other hand, various proposals as to photosensitive polyimide resins the solubility of which varies upon exposure to light have been made in recent years. Typical examples of such a photosensitive polyimide resin include compounds with a photopolymerizable acryloyl group introduced into a polyamic acid (hereinafter may called polyamide acid) which is a precursor of a polyimide resin (Japanese Patent Publication Nos. 30207/1980 and 41422/1980, etc.). A precursor of polyimide, in which an acryloyl group has been introduced in the form of a salt, has also be proposed (Japanese Patent Publication No. 52822/1984).
In the photosensitive polyimide resin making use of such a polyimide precursor, a film of the photosensitive polyimide resin is formed on a semiconductor device, a pattern is formed by irradiation of light, and the polyimide resin is cyclized by a heat treatment, thereby completing polyimidation. At this time, the moiety of the photosensitive group is eliminated and volatilized, thereby reducing the final thickness of the film formed.
The use of the photosensitive polyimide resin permits marked shortening of a pattern-forming process. However, the conventional photosensitive polyimide resins have involved drawbacks such as the synthesizing route is complicated, the photosensitive group is not fully eliminated by polyimidation, or they are insufficient in storage stability and sensitivity to light.
There have recently been proposed photosensitive resin compositions comprising an actinic ray-sensitive functional group (Japanese Patent Application Laid-Open Nos. 70661/1992 and 77741/1992). These photosensitive resin compositions have been prepared by a special process in which a polyamic acid having carbon-carbon double bonds at its terminals is reacted in a solvent composed of an amide compound having a polymerizable carbon-carbon double bond, and are characterized in that the polyamic acid itself is photosensitive and the solvent itself is 100% photosensitive. However, the polyamic compounds disclosed in these publications are easy to eliminate their modified terminal groups and poor in stability. This polyamic compound is generally obtained by modifying a part of the acid anhydride with the actinic ray-sensitive functional group to form an ester and then adding a diamine to form a polymer. However, this process can form only a polyamic acid wide in scattering of molecular weight and hence encounters difficulties in synthesizing only a polyamic acid high in molecular weight. In the photosensitive resin compositions described in these publications, the solvent also functions as a photosensitive auxiliary. Therefore, such a composition has involved a problem that when the composition is applied to a substrate and subjected to a heat treatment to form a film, a residual stress of the film against the substrate increases to a great extent.
On the other hand, the photosensitive polyimide resin compositions have involved a problem that they are difficult to develop with an alkaline developer or alkaline aqueous solution. For example, the negative type photosensitive polyimide resins disclosed in Japanese Patent Publication Nos. 30207/1980 and 41422/1980, etc., and those obtained by introducing a photopolymerizable group through an ionic bond into a precursor of polyimide disclosed in Japanese Patent Publication No. 52822/1984 and the like all use organic solvents as their developers. Therefore, such photosensitive polyimide resins have involved such problems that developer cost is high, explosion-proof apparatus are required, and human health may be endangered.
As alkali-developed or alkaline aqueous solution-developed photosensitive resins, there have hitherto been reported, for example, those obtained by introducing a protective group capable of leaving into a carboxyl group or a phenolic hydroxyl group under acid or alkaline conditions J. Macromol. Sci. Chem., A24, 10, 1407 (1987), Japanese Patent Application Laid-Open No. 259351/1989 and Japanese Patent Application Laid-Open No. 363361/1992!. However, these resins require to introduce a substituent group great in steric hindrance through a covalent bond into a main-chain carboxyl group or side-chain hydroxyl group. Therefore, they involve such problems that their synthetic processes become complicated, they require a high curing temperature, and film properties of films formed therefrom are deteriorated.
It has been proposed to incorporate a dihydropyridine derivative, which is a photo-induced base generator, and accelerate the imidation-facilitating effect of a base formed by exposure by a PEB (post-exposure baking) treatment, thereby developing a difference in solubility caused by the imidation with an alkaline developer (Japanese Patent Application Laid-Open Nos. 5995/1993 and 281717/1993). However, these processes involves such problems that the PEB treatment must be used, and the film formed has a low sensitivity to exposure, and so an exposure value must be made greater.
It has also been proposed to use an alkaline developer or alkaline aqueous solution as a developer for photosensitive resin compositions added with a photopolymerizable monomer as a crosslinking auxiliary (Japanese Patent Application Laid-Open Nos. 183439/1988 and 100424/1993). In these processes, special crosslinking monomers such as those having an isocyanate structure or a urethane structure are added as the photopolymerizable monomers. Since the base polymers have no photopolymerizable group in their structures, there is a problem that sensitivity and resolution are insufficient.